Got a piece of Sapphire Radeon HD 5850 yesterday and had some test.

Ubuntu Maverick x64 in default only supports KMS and no XV, no EXA, on OpenGL, no PowerSaving

Added xorg-edgers to repository and did an update, all works!

KMS: works
XV: works, tear free video playback
EXA: works
OpenGL: works, v-synced Compiz and simple GL applications, WebGL on Chromium, some texture corruptions though
PowerSave: works, echoing "low" to /sys/class/drm/0/power/power_profile results in card drop to 175 Core, 300 Mem, 0.95v and dead silent
Temperature via LM-Sensors: works, getting GPU temp idling at 45C

I am overall very happy with the result. Open source driver with almost* everything I want on an Evergreen GPU. Good work!

Almost = OpenGL is still very minimally supported but for the timeframe I couldn't ask for more I guess.